Something like this would best work out for middle of the road and older, cheaper games. Like if the game store is only going to pay you $5 or $7 for a game. But if the game is new, and the stores are going to pay a fairly high price, very few people if anyone on this site is going to give you anymore for it. A little while back I wanted to sell off a splinter cell pandora tomorrow, riddick, and something else new, and I got fair prices (like around 25-30 for each game) plus 30% extra for trading in 3 games. I am not complaining because I am the same way but usually on this site no one would ever really pay more than $25/30 for a used game, because you can always find the game on sale somewhere for 40, then someone can price match, GGC, etc. etc. You also have to keep in mind that it's sometimes easier to go into a store than deal with someone on the internet who could potentially rip you off (it happens.)
So yeah definately try to sell to others rather than getting nothing from GS/EB, but if the game is new enough you can get a fair price from them.